Fifth in an occasional series profiling US Olympic hopefuls training for the Summer Games in London.

It’s not as though he was the first guy to splash around in Walden Pond. Henry David Thoreau paddled across it, generations of skinny-dippers have immersed themselves, and triathletes train there. But when Alex Meyer does his extended up-and-backs at the Concord swimming hole, he eventually attracts a cadre of the curious.

“They’ll look at me like I have two heads,’’ said the 23-year-old Harvard graduate. “What’s this guy doing?’’

Just getting in some outdoor work while he preps for this summer’s Olympics in London, where Meyer will be the only US male in the 10-kilometer open-water swim. Because of the recent unseasonable conditions, he hasn’t been making the trek to Walden. He has had to use the “endless pool’’ - essentially a chlorinated treadmill where Meyer can go nowhere fast for three hours at a time - at his alma mater’s indoor facility next to the river.

He and his fellow open-water swimmers inhabit the wild side of the sport. While the Phelpses and Coughlins are doing flip turns in natatoriums, their alfresco counterparts are competing in rivers and harbors and lakes. Their shortest event - 5 kilometers - is more than three times longer than anything indoors.

“We’re very proud of what we do,’’ said Meyer, the former 25K world champion. “Sprinters get all the fame, they get all the glory. But we know that we’re badder than anybody else is. We’re happy with that, embracing that culture, the lifestyle of an endurance athlete.’’

Open water is NASCAR in neoprene caps, complete with drafting and bumping.

“There’s always something going on,’’ said Harvard swimming coach Tim Murphy (not to be confused with the Crimson’s pigskin version), who will coach the US competitors in London.

“You’re never in a lane by yourself. There are people ahead of you, people behind you, people to your right and left. You’re trying to get in, you’re trying to get out. You’re trying to make a break, you’re trying to back off.’’

That style of Olympic swimming dates back to the first modern Games in 1896, when all of the events were held in the icy waters off the Athenian port of Piraeus. But open water wasn’t added to the program until four years ago.

“It’s a young sport,’’ said Meyer. “Not that many people even know that it’s in the Olympics now.’’

Source of inspiration

The only time open water makes headlines is after a tragedy, as when Fran Crippen, Meyer’s mentor and former road roommate, vanished not far from shore at the World Cup event two autumns ago in Dubai and was found dead after Meyer led the search.

“I don’t think there’s really ever going to be a time where I’m completely at peace with it, but it’s gotten better over the past year,’’ said Meyer, who since has been an outspoken advocate for better safety measures.

Had Crippen lived, Meyer is convinced they would have been teammates at Olympus. So he plans to swim for both of them in London.

“I see it as more of a motivator than a burden,’’ said Meyer, who has grown close to Crippen’s family in the Philadelphia area. “I feel like there are so many reasons that I want to do well, and obviously for all of Fran’s family and friends and fans, that’s just added incentive for me to have a really good performance.’’

Meyer made the team - the first swimmer to qualify - by finishing fourth at last year’s world championships near Shanghai, where the water temperature was sauna-like. The water conditions were so unhealthy for the 25K - 90 degrees by the end of the race - that Meyer chose not to defend his title.

“It was absolutely inexcusable,’’ he said. “They were basically the same exact conditions that Fran died in less than a year before, and now they’re having a 25K in the same conditions with higher stakes.

“The thing that just drives me nuts is that FINA [the international aquatics federation] made this recommendation of 31 degrees [87.8 Fahrenheit] being the max and they blow that off.

“Of the 35 guys that entered, 19 of them finished the race. If just a little more than half of the people finish the race, there’s something clearly wrong with the event.

“They were pulling guys out, guys quit on their own. At one point, I got so furious that I walked out onto the dock and I was pulling FINA people aside and saying, ‘What are you doing? This is out of control. Do you not remember what happened the last time that you did this? You can’t be having the race go on.’ ’’

It’ll likely be more than 20 degrees cooler in The Serpentine inside London’s historic Hyde Park, where the technical nature of the course should suit Meyer’s relatively smallish build (5 feet 11 inches, 155 pounds) and ability to work in close quarters.

What was attractive to him about open water, other than its absence of walls, was the presence of elbows and knees.

“I’ve always secretly coveted playing contact sports,’’ said Meyer. “Open water in a way kind of fills that void a little bit.

“It’s not like it was a burning desire to tackle people. I’ve grown up doing long, exhausting workouts, so swimming a 10K comes naturally to me because it’s basically what I do every day.’’

High pain tolerance

What most people would view as unendurable tedium, Meyer, who majored in human evolutionary biology, considers a run-of-the-treadmill workout. His capacity for pain was legendary at Harvard, where he was just warming up when his teammates in the distance lane were backing off.

“Alex would have an almost insane smile on his face, this maniacal grin as the sets got harder,’’ recalled former Crimson captain Sam Wollner, who competed with Meyer in the 400 and 1,500 meters at the 2008 Olympic trials. “He had just the right screws loose to be an elite distance swimmer.’’

If anything, Meyer found the metric mile too short. He’d been drawn to open water since he was a kid in upstate New York, goofing around at Lake George. When his Ithaca club coach told him about a woman who swam the 40-mile length of Cayuga Lake, Meyer was entranced.

“It was one of the first things that put open water in my head,’’ he said. “I thought it was the coolest thing ever.’’

By 14, Meyer was turning up at national championships, but he essentially was still a pool swimmer. Endless laps at Blodgett Pool left him uninspired, so he biked out with Wollner one day to check out what Thoreau called a “distiller of celestial dews.’’

“As much as I liked Blodgett, that place is a dungeon,’’ said Wollner. “No windows. It might as well be a bomb shelter.

“Walden is the exact opposite. It’s open. It has that Thoreau majesty to it.’’

Meyer soon was hooked on the outdoor game and made it to the 2009 world championships in Rome after his junior year. But his craving for dry-land contact almost ended his career that autumn when he broke his back after a bit of what he calls “Frisbee wrestling’’ with a teammate inside the football stadium.

“It was mostly embarrassing, because the recruits were with us,’’ said Meyer. “It was honestly the most pain I’ve felt in my entire life.

“That was awful. I couldn’t get off the ground for half an hour. I fell down and I’m lying there writhing in pain and everyone is standing around me in a circle.’’

For a while, Meyer worried that the injury, which turned out to be a compression fracture, would wipe out his senior season. As it was, he wore a back brace for six weeks but came back to make All-America at 1,650 yards in the NCAAs.

The following summer in Quebec, he won the 25K world title, prevailing by one second over Italy’s Valerio Cleri in a duel that consumed more than 5 1/2 hours.

The battle within

“There’s always a fight-or-flight tug-of-war going on within,’’ said Meyer. “The longer the race goes on, the more the chances there are that the fighting instinct is going to slip away a little bit.

“One of the things I was most proud of was not just because I won but because I felt there were times where I felt myself being pulled very strongly in the opposite direction mentally than I should be going.

“But I managed to reel it back in and stay positive and focused and keep that winning mentality, and it paid off. That internal battle was something that I’d never really experienced to that degree before.’’

The 10K, which is the only Olympic event, presents a decidedly different trial.

“The 25K is a much more challenging test of mental endurance and patience and spending your energy efficiently over a long period of time,’’ said Meyer. “The 10K is a little more about tactical positioning, making decisions at the right time and place, taking advantage when there’s an opportunity that may open and shut very quickly.’’

Even in a two-hour race, victory can come down to a couple of strokes.

“There’s a certain amount of volatility in the sport that just comes naturally,’’ said Meyer. “It’s not quite like short-track speedskating where you never know who’s going to come out on top. There’s a little bit of that — a little luck, a little chance — but that’s just the nature of it and we all embrace that.’’

Adaptability is indispensable in a sport where weather plays such a whimsical role.

“You go in with a plan but you’ve got to have some flexibility and mental toughness to deal with the adversity because it’s going to happen,’’ said Murphy. “It’s part of the game.’’

Not all of the adversity happens in the water. Riding his bicycle home from the store in mid-January, Meyer went over the handlebars when the chain came off and he broke his collarbone.

“More of a mechanical malfunction,’’ said Meyer, who had a screw inserted in the bone during surgery and was out of the pool for two months, missing the first two World Cup events in South America.

Had Meyer been a pool swimmer, he might not have been able to return to top form in time for the Olympic trials at the end of next month, so having his London ticket already in his pocket was a lovely bit of fortune.

It allowed him to use last month’s national championships in Fort Myers to get in some racing, and he’ll compete in the World Cup in Quebec at the end of July to sharpen up.

Meyer is still going to the Olympic pool trials, though. That’s where the entire swim team will be announced, and he’d like to partake in the festivities.

He’ll also take the blocks in the 1,500, if only to prove that the open-water guys can handle flip turns.

“There’s a lot of people who think we can’t make it in the pool,’’ said Meyer. “It’d be really cool if I can go to Omaha and show that I can throw down a legit mile.’’